The role of polymer additions in concrete
Seth Price | January 20, 2025Concrete has, quite literally, been the building block of civilization for many years. Traditional concrete consists of some proportion cement, aggregate (stones and sand) and water, though the ratios vary with time and place. Furthermore, the size distribution of the aggregates had an impact on the overall strength of the concrete, and so different cultures and regions used different local materials to develop their specific type of concrete.
Even with all of the history behind concrete, its composition is still being investigated and altered in modern times. One of the more recent developments is the addition of polymers in concrete. Polymers can increase the strength, make the concrete more “green” and can decrease the mass of the final product. With these advantages in mind, material scientists are exploring all of the possible additives and how they may impact the structures of the future.
Precast concrete. Notice the varying texture and porosity, all a function of working with concrete. Source: Ablazejo/CC BY-SA 3.0
Increased strength
At first glance, the addition of polymers into concrete does not seem like it would strengthen the bulk material. However, there are several ways in which polymer-laden concrete can be strengthened.
The addition of polymer fibers can provide a little extra support, should the material be strained in tension or torsion. While concrete is strong in compression, it is much weaker in tension or shear. Rebar steel is the current choice for strengthening concrete in tension, as the steel “fibers” can bear some of the tensile load.
A recent review paper on fiber-reinforced concrete highlights some of the key advantages, such as increased strength, fracture toughness and thermal stress reduction. Polypropylene fibers have been shown to increase the strength, by distributing stress and preventing crack tips from growing. Unlike steel rebar, the polypropylene fibers do not rust, improving the lifetime of the structure.
Another strengthening mechanism is the increased adhesion strength between aggregates and cement due to polymer additions. One of the limiting factors of concrete is the ability for a crack to propagate around poorly bonded materials. A stone that is not bonded well to the surrounding matrix is a major liability for reduced strength. Another recent article has shown that several different additives can increase the bonding and bridging between particles, preventing these cracks from propagating.
Mounds of waste plastic. The reality is that much of what is sent to plastic recycling facilities is either discarded in the oceans or burned as fuel. Source: Dying Regime/CC BY 2.0
Recyclability and sustainability
Perhaps the biggest driving force behind adding polymers to concrete is that there is plenty of waste plastic. One thought is that the addition of shredded plastic bottles could be added to concrete and solve two issues at once: what to do with all the waste plastic, with the added benefit of strengthening the concrete. Only a small percentage of plastic is actually recycled. It is simply not economical to do so. Therefore, adding plastic to concrete may be a way to handle some of this waste material.
Concrete production is a notoriously large source of greenhouse gas generation. If some fraction of the concrete can be made from already-existing materials, the greenhouse gas production will be reduced. It is worth noting that greenhouse generation cannot be eliminated by polymer additions, as the cement must be calcined at high temperatures. However, if some cement is replaced with polymer, the net result is lower carbon emissions.
Decreased mass
One slight advantage of polymeric additions is the decrease in density, thus leading to a decrease in mass for objects made from polymeric concrete. This decrease in mass must be considered against the other design requirements, but lower mass means lighter bridges, building floors. These lighter structures lead to fewer, lighter support structures. It also opens up the possibility of newer designs that do not have to meet the old “rules of thumb,” such as placing vertical supports as frequently.
Also, the addition of some foaming polymers can add non-connected pore space. The pores do not make the concrete any stronger, but they can improve its insulating properties. Porous concrete can help reduce heating and cooling loads in buildings by rooms insulated from the elements. The pores also help dampen vibration and sound, creating quieter factory floors and retail and office spaces.
Potential limitations
While there are a few advantages to polymer additions in concrete, there are some challenges that must be mastered. Depending on the amount and type of polymer added, the concrete can become much more difficult to work with. It can change the viscosity and make it harder to spread. It can also affect the curing times, which could be a limitation for some pumping and transportation concerns. It could also be an advantage for 3D-printed concrete structures.
The addition of polymers to the concrete mix can increase the cost. While this can be alleviated a little by using waste materials, such as shredded waste plastic, it is hard to get much cheaper than sand. The increased cost may be a barrier for some construction projects.
The complexity of the polymeric systems is still being investigated. Polymers do not rust like steel rebar does, but they can degrade under ultraviolet light, are prone to wear and have other chemical and thermal degradation problems. Overall, the long-term properties in concrete are not well understood at this time.
Final thoughts
While concrete has been around for centuries, there is never going to be an end to its refinements. As with most materials, there will always be a flaw that needs to be improved, or a property that can be expanded upon, especially given a new use or context. The use of polymer additives has been ongoing for a few years and will continue for ages to come.